It is well known that urbanization and industrialization have resulted in the rapidly increasing emissions of volatile organic compounds (VOCs), which are a major contributor to the formation of secondary pollutants (e.g., tropospheric ozone, PAN (peroxyacetyl nitrate), and secondary organic aerosols) and photochemical smog. The emission of these pollutants has led to a large decline in air quality in numerous regions around the world, which has ultimately led to concerns regarding their impact on human health and general well-being. Catalytic oxidation is regarded as one of the most promising strategies for VOC removal from industrial waste streams. This Review systematically documents the progresses and developments made in the understanding and design of heterogeneous catalysts for VOC oxidation over the past two decades. It addresses in detail how catalytic performance is often drastically affected by the pollutant sources and reaction conditions. It also highlights the primary routes for catalyst deactivation and discusses protocols for their subsequent reactivation. Kinetic models and proposed oxidation mechanisms for representative VOCs are also provided. Typical catalytic reactors and oxidizers for industrial VOC destruction are further discussed. We believe that this Review will provide a great foundation and reference point for future design and development in this field.

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Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources.

Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and ~3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

https://science.sciencemag.org/content/sci/328/5975/224.full.pdf

Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects

The article discusses oxidation catalysis by substitutional cation doping of binary oxides. Substitutional cation doping is not the only possibility. One can imagine that replacing some anions with other anions may also be beneficial. There is evidence that the presence of small amounts of halogen in the feed or on the oxide surface improves its catalytic activity. It is very likely that doped oxide catalysts have been used before the concept was formulated explicitly. Most oxide catalysts have low levels of impurities that may be substitutional dopants. If they segregate at the surface, they can affect the catalytic activity without our knowledge even though their net concentration is very low. It is also possible that the 'as-prepared' catalyst is a doped oxide that, under reducing reaction conditions, is converted to very small metallic dopant clusters supported on the host oxide. The physical and chemical properties of such clusters are different from those of a bulk metal, and it is difficult to distinguish them from a doped oxide.

Catalysis by doped oxides.

Composite Titanium Dioxide Nanomaterials

Different manganese oxide phases were prepared as thin films to elucidate their structure–function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temperatures (to improve film adherence and crystallinity) were tested in neutral and alkaline electrolytes. Differential electrochemical mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of crystalline α-Mn2O3 films was determined to onset at an overpotential (η) of 170 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE. Amorphous MnOx films heated at 573 K (MnOx-573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystallization detected by Raman spectroscopy. The onset of water oxidation of MnOx-573 K films was identified at η = 230 mVRHE (at...

http://pubs.acs.org/doi/pdf/10.1021/jp500939d

Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water

Ce 1− X Ni X O 2 oxides with X varying from 0.05 to 0.5 were prepared by different methods and characterized by XRD and TPR techniques. Ce 0.7 Ni 0.3 O 2 sample prepared by sol–gel method shows the highest reducibility and the highest catalytic activity for methane combustion. Three kinds of Ni phases co-exist in the Ce 1− X Ni X O 2 catalysts prepared by sol–gel method: (i) aggregated NiO on the support CeO 2 , (ii) highly dispersed NiO with strong interaction with CeO 2 and (iii) Ni atoms incorporated into CeO 2 lattice. The distribution of different Ni species strongly depends on the preparation methods. The highly dispersed NiO shows the highest activity for methane combustion. The NiO aggregated on the support CeO 2 shows lower catalytic activity for methane combustion, while the least catalytic activity is found for the Ni species incorporated into CeO 2 . Any oxygen vacancy formed in CeO 2 lattice due to the incorporating of Ni atoms adsorbs and activates the molecular oxygen to form active oxygen species. So the highest catalytic activity for methane combustion on Ce 0.7 Ni 0.3 O 2 catalyst is attributed not only to the highly dispersed Ni species but also to the more active oxygen species formed.

Reduction property and catalytic activity of Ce1-XNiXO2 mixed oxide catalysts for CH4 oxidation

Total oxidation of chlorinated aromatics on supported manganese oxide catalysts was investigated. The catalysts have been prepared by wet impregnation method and characterized by XRD and TPR. Among the catalysts with the supports of TiO2, Al2O3 and SiO2, titania-supported catalyst (MnOx/TiO2) gives the highest catalytic activity. MnOx/TiO2 (Mn loading, 1.9 wt.%) shows the total oxidation of chlorobenzene at about 400°C. The activity can be stable for over 82 h except for the first few hours. At lower Mn loadings for MnOx/TiO2, only one reduction peak appears at about 400°C due to the highly dispersed manganese oxide. With the increase of Mn loading, another reduction peak emerges at about 500°C, which is close to the reduction peak of bulk Mn2O3 at 520°C. TPR of the used catalyst is totally different from that of the fresh one indicating that the chemical state of the active species is changed during the chlorobenzene oxidation. The characterization studies of MnOx/TiO2 showed that the highly dispersed MnOx is the precursor of the active phase, which can be converted into the active phase, mainly oxychlorinated manganese (MnOyClz), under working conditions of chlorobenzene oxidation.

Catalytic oxidation of chlorobenzene on supported manganese oxide catalysts

A series of CexTi1-xO2 mixed oxide materials was synthesized by a sol−gel method with x varying from 0.1 to 0.9 and characterized by XRD, Raman, and TPR techniques. The structure of the mixed oxide...

Structure and Redox Properties of CexTi1-xO2 Solid Solution

Propylene epoxidation by air was carried out on nacl-modified silver (nacl/ag) catalysts, and the catalysts were characterized by x-ray diffraction (xrd) and x-ray photoelectron spectroscopy (xps). the effects of nacl loadings, propylene to oxygen ratio, and the reaction time on the catalytic performance were investigated. it was found that the addition of nacl to silver significantly increases the propylene oxide (po) selectivity. the po yield has a maximum when the nacl loading is about 10 wt.%. also 12.4% conversion of propylene and 31.6% selectivity to po are obtained on the nacl/ag (10 wt.%) catalyst at 350 degreesc, space velocity 1.8 x 10(4) h(-1) and c3h6:o-2 = 1:2. xps and xrd characterizations show that agcl formed on the silver catalyst was favorable to propylene epoxidation. a compound with highly oxidized ag ion was also found, which may be effective for the reaction. (c) 2002 elsevier science b.v. all rights reserved.

Mengfei Luo

Papers

Reduction property and catalytic activity of Ce1-XNiXO2 mixed oxide catalysts for CH4 oxidation

Catalytic oxidation of chlorobenzene on supported manganese oxide catalysts

Structure and Redox Properties of CexTi1-xO2 Solid Solution

Epoxidation of propylene on NaCl-modified silver catalysts with air as the oxidant

Oxidative Destruction of Chlorobenzene and o-Dichlorobenzene on a Highly Active Catalyst: MnOx/TiO2–Al2O3